Maximizing the areal density of recording is necessary for miniaturization and for lowering the cost per bit of computer data-storage technology. Reducing the head-to-medium spacing, or the flying height, is one of the more effective ways to achieve a higher density on a rigid disk. However, to have read/write heads flying over the disk surfaces at submicrometer spacing, the topography of the disk surface must be very uniform and the textured nickel-phosphorus aluminum (NiP/Al) substrates used in most media do not meet this requirement. Therefore, a different substrate is needed to provide a better surface allowing the head to fly closer to the disk.
Glass and ceramic substrates are potential candidates for such an application, as the rigidity of these materials is advantageous in making thinner disk substrates and the hardness eliminates the need for electroless NiP plating. Most importantly, glass and ceramic provide a superior smoothness and flatness which are not achievable on the current NiP/Al substrates.
However, the magnetic recording properties of media prepared on a glass or glass-ceramic substrate have been found to be inferior to media prepared on a NiP/Al substrate (Tsai, 1993). In particular, the coercivity of media formed on glass substrates is low, often less than 1000 Oe (Fisher).
It is well known that the chromium underlayer in longitudinal recording media significantly enhances the coercivity of cobalt alloy films (Lazzari). Structural analyses have shown that cobalt alloy films can grow epitaxially on chromium under appropriate conditions. As a result, the crystallographic orientation and grain size, which directly influence the magnetic recording properties, of cobalt alloy films are determined primarily by the chromium underlayer (Laughlin, Mirzamaani, Chen).
Similarly, the crystallographic orientation of the chromium underlayer, which influences the overlying magnetic layer and the ultimate recording properties, is related to the substrate material. On NiP/Al substrates, the crystallographic orientation Cr(200) normally occurs. However, on glass-ceramic substrates a Cr(110) orientation is observed (Tsai, 1992). The Cr(200) orientation has been shown to give epitaxial growth for most cobalt alloy media resulting in media with a high coercivity and good recording properties (Tsai, 1992, Duan, Lal, Pressesky).
Because of the advantages of glass or glass-ceramic as a substrate, various approaches to improving the recording properties of media formed on non-metallic substrates have been reported. For example, sublayers of NiP (Ishikawa), Ti (Kogure), and Cr (Hedgcoth, Tsai 1991) between the chromium underlayer and the glass substrate have been applied. Alternatively, application of a bias, DC or rf, to the substrate has been investigated as a means to improve the recording properties of media formed on a glass substrate (Fisher, Ishikawa).